Dark colored electroceramic coatings for magnesium

ABSTRACT

This invention relates to articles having magnesium-containing metal surfaces with a black, brown or bronze electroceramic coating, chemically bonded directly to the magnesium metal surfaces, the coating having an outer darkly colored layer and an underlying interfacial layer. Articles having a composite coating comprising first sectors of the electroceramic coating and second sectors comprising organic and/or inorganic components different from the electroceramic coating are also provided. The invention further relates to processes of making and using the articles.

FIELD OF THE INVENTION

This invention relates to articles having metal surfaces of magnesium which have been provided with a dark colored, electroceramic coating that is chemically bonded to the metal surfaces, desirably a black, brown or bronze electroceramic coating. As used herein, “dark colored” means black, brown, bronze or gray. Articles having a composite coating comprising first sectors of the electroceramic coating and second sectors comprising organic and/or inorganic components different from the electroceramic coating are also provided. The invention further relates to processes of making and using the articles.

BACKGROUND OF THE INVENTION

The light weight and strength of magnesium (˜1.74 gm/cm³ density) and alloys relative to ferrous metals makes products fashioned therefrom highly desirable for use in manufacturing parts, for example, electronic devices, including handheld electronic devices; motor vehicles; aircraft and other products where low density is beneficial.

One of the most significant disadvantages of magnesium or magnesium alloy is susceptibility to corrosion, generally taking place on Mg in the presence of oxygen, moisture and other environmental agents, such as human fingerprint constituents. A variety of coating products have been used on magnesium or magnesium alloy surfaces to provide them with desired dark colors and attempting to improve corrosion resistance, but none has met needs for both corrosion resistance and dark color.

One method used to improve corrosion resistance of metal surfaces is anodization, where a metal (M) surface is oxidized electrically to form a metal oxide (MOx) layer from molecules of the metal surface, see for example U.S. Pat. No. 4,978,432 and U.S. Pat. No. 5,264,113. Anodization of magnesium or magnesium alloy affords some protection against corrosion, but U.S. Pat. No. 5,683,522, indicates that conventional anodization often fails to form a protective layer on the entire surface of a complex workpiece, and can contain cracks, some down to the metal surface, at sharp corners. This lack of coverage negatively affects corrosion, and also fails to provide a uniform colored surface.

Plasma Electrolytic Oxidation (PEO), also known as Micro Arc Oxidation (MAO), Spark Anodizing and Microplasma Oxidation, referred to herein collectively as “PEO”, is a process in which the surfaces of certain metals, e.g. aluminum and magnesium, are converted into oxide coatings using high-voltage alternating current applied to a metal part submerged in an electrolytic bath. PEO is characterized by intense sparking due to micro-arc discharges which break down initially deposited oxide layers. The discharges leave “craters” on the surface of the growing coating having an average diameter of more than one micron after 1 min and more than two microns after 30 min. The surface roughness also increases as the PEO coating increases in thickness, which is often undesirable.

PEO processing of magnesium or magnesium alloy produces a crystalline oxide (60-80 vol. %) coating with minor amounts of silicates and/or phosphates, depending on the content of the PEO bath. PEO processes have disadvantages including lack of uniformity in the coating structure between crystalline and amorphous, cratered areas, and thicker and thinner coating areas which can negatively impact color uniformity making it unsuitable for show surfaces. Further, PEO produces a brittle sub-layer with a porosity of more than 15%, which is removed by an additional polishing step. Polishing has the disadvantages of additional processing and, often, manual labor, as well as loss of dimensional integrity of the article and challenges in uniformly polishing complex articles or those having non-uniform coating layers due to throw-power limitations of PEO. More importantly, colored features of the coating can be negatively affected, e.g. made uneven, by removal of coating layers containing color.

A disadvantage of show surfaces of coated magnesium, e.g. casings for electronic devices, is their susceptibility to marring, corrosion and, particularly for dark colored surfaces, fingerprinting, which increases manufacturing cost through efforts to reduce marring, corrosion and fingerprinting, usually by more layers of coating.

It is desirable to provide a process for uniformly coating Mg alloys with dark colored coatings and coated Mg alloy articles with a dark colored coating that provides improved corrosion resistance.

SUMMARY OF THE INVENTION

At least some of the drawbacks described above are reduced by the invention described herein.

It is an object of the invention to provide an article having at least one metal surface of magnesium or magnesium alloy having a layer of an inorganic-based, desirably electrolytically deposited, coating chemically bonded directly the at least one metal surface and exhibiting a black, brown, bronze or gray colored appearance to the unaided human eye.

The inorganic-based coating may have additional layers deposited thereon, may form a composite coating comprising the inorganic-based coating and a second component distributed throughout or in contact with at least a portion of the inorganic-based coating and/or the coating on the magnesium or magnesium alloy surface may comprise a reaction product the inorganic-based coating and a second component.

It is also an object of the invention to provide a method of depositing a dark colored coating that improves corrosion resistance of magnesium or magnesium alloy metal substrates comprising:

-   A) providing an alkaline electrolyte, which may be a solution or     dispersion, comprising water, an organic amine, a source of     phosphorus, and one or more additional components selected from the     group consisting of: water-soluble transition metal oxides,     water-soluble transition metal salts and mixtures thereof; and a     cathode in contact with the alkaline electrolyte; -   B) placing an article having at least one bare metallic magnesium or     magnesium alloy surface in contact with the electrolyte and     electrically connected thereto such that the surface acts as an     anode; -   C) passing a current between the anode and cathode through the     electrolyte solution for a time effective to generate a first layer     of an inorganic-based coating chemically bonded directly to the bare     metal surface, the first layer appearing black, brown, bronze or     gray to the unaided human eye; -   D) removing the article having the first layer of an inorganic-based     coating from the electrolyte and optionally drying it; -   E) optionally post-treating at least the first layer of the     inorganic-based coating by     -   1) contacting the first layer of the inorganic-based coating         with a post-treatment composition different from the         inorganic-based coating, the post-treatment composition         optionally being reactive with the inorganic-based coating;         and/or     -   2) after step 1) if present, applying to the first layer of the         inorganic-based coating, a polymeric composition thereby forming         a second layer comprising organic polymer chains and/or         inorganic polymer chains having a layer thickness of 0.1 micron         to 15 microns; and -   F) optionally applying additional protective layers, e.g. lacquer,     paint, after the post-treating step.

It is an object of the invention to provide a method wherein the post-treating step E) is present as a step of contacting the first layer of inorganic-based coating with a second component different from the inorganic-based coating; distributing the second component throughout at least a portion of the first layer, in particular the pores; and depositing a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating,

It is an object of the invention to provide a method wherein step E) i) is present and comprises a step of introducing at least one Ti, Zr, Hf or combinations thereof-containing composition as the second component to the second sub-layer of inorganic-based coating, contacting at least the external surfaces and desirably at least some of the internal surfaces of the second sub-layer, whereby the second component forms a thin inorganic-based coating in contact with the external surfaces of the inorganic-based coating and lining at least a portion of the pores in the inorganic-based coating. It is an object of the invention to provide a method wherein step E) 1) comprises reacting the composition and elements of the inorganic-based coating to thereby form a portion of the second component, which is different from the inorganic-based coating and the composition.

It is an object of the invention to provide a method wherein step E) ii) is present and comprises contacting the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and optionally applying a layer of paint after the post-treating step.

It is a further object of the invention to provide a method that is performed in the absence of any step prior to step B) that deposits materials containing the element silicon, e.g. silicates, and/or fluorine, e.g. metal fluorides, nonmetal fluorides, fluorometallates, on the metal surface. In some embodiments, neither Si nor F are present in the electrolyte or the coating, other than trace amounts from the metal substrate and electrolyte raw materials.

It is an object of the invention to provide a method comprising controlling temperature and concentration of the electrolyte and time and waveform of the current in step C) such that the inorganic-based coating is 1-50 microns, preferably 1-20 microns, in thickness and comprises carbon, oxygen, phosphorus, one or more transition metals, and magnesium. It is a further object of the invention to provide a method wherein forming the first layer in step C) utilizes less than 10 kWh per square meter, measured as square meters of total metal surface being coated.

It is an object of the invention to provide a method further comprising performing at least one step selected from cleaning, etching, deoxidizing, desmutting, and combinations thereof prior to placing the magnesium containing article in contact with the alkaline electrolyte such that prior to generating the first layer, from 0.05 to 50 g/m2 of metal is removed from the bare metallic magnesium or magnesium alloy surface.

It is an object of the invention to provide a method further comprising a step of masking a portion of the magnesium containing article prior to placing the at least one metallic magnesium or magnesium alloy metal surface in contact with the alkaline electrolyte.

It is an object of the invention to provide a method wherein after step C), no more than 10 mg/m2 of the inorganic-based coating is removed.

It is an object of the invention to provide a method wherein said current is pulsed direct current having an average voltage in a range of 50 to 700 volts.

It is an object of the invention to provide a composition of matter suitable for use as an electrolyte in a plasma electrolytic deposition bath. The electrolyte may be an alkaline electrolyte, which may be a solution or dispersion, which comprises; desirably consists essentially of; or optionally consists of water, an organic amine, a source of phosphorus, and at least one water soluble source of at least one transition metal, e.g. one or more additional components selected from: water-soluble transition metal oxides, water-soluble transition metal salts and mixtures thereof. In one embodiment, the organic amine is monoethanolamine and the at least one transition metal element comprises one or more of iron, vanadium and tungsten. In one embodiment, the alkaline electrolyte contains less than 100 ppm silicon or aluminum and is essentially free of fluorine and tertiary amines. In one embodiment, the organic amine is a primary monoamine in the absence of cyclic amines, and the at least one transition metal element consists of iron or vanadium or tungsten. In one embodiment, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and vanadium and the alkaline electrolyte has a pH of at least 10.2. In one embodiment, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises tungsten. In one embodiment, the alkaline electrolyte is vanadium free, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and optionally a second transition metal element other than vanadium. In one embodiment, the organic amine is a primary monoamine in the absence of cyclic amines, and the at least one water-soluble or dispersible source of at least one transition metal element comprises iron citrate.

The composition may be provided as a storage-stable two pack system wherein Part A contains water; a source of phosphorus, for example phosphoric acid, phosphorous acid, pyrophosphate, phosphonate; one or more water soluble salts of transition metals, for example iron, vanadium, tungsten and the like; wherein the mass ratio of phosphorus to total amount of transition metal is 4:1 to 1:1; and Part B contains organic amine, preferably monoethanolamine, Part A and Part B being provided in amounts such that the mass ratio of Part A to Part B be ranges from 1:1 to 2:1.

It is an object of the invention to provide an article comprising at least one magnesium or magnesium alloy metal surface coated according to a method disclosed herein.

It is an object of the invention to provide an article comprising at least one metallic magnesium or magnesium alloy surface coated with a dark-colored first layer of an inorganic-based coating chemically bonded directly to the at least one metallic magnesium or magnesium alloy surface wherein the inorganic-based coating has a bilayer structure, comprising: a first sub-layer directly bonded to the metallic magnesium or magnesium alloy surface at a first interface, said first sub-layer comprising Mg, O, C, P and at least one transition metal element; a second sub-layer integrally connected to the first sub-layer at a second interface, said second sub-layer comprising external surfaces at an outer boundary of the inorganic-based coating, and optionally internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, said second sub-layer comprising the Mg, O, C, P and at least one transition metal element; wherein the weight percent of C in the second sub-layer is greater than that of the first sub-layer. In one embodiment, the weight percent of C in the second sub-layer exhibits an increasing concentration gradient from the second interface to the external surfaces of the inorganic-based coating.

It is an object of the invention to provide an article having at least one metallic magnesium or magnesium alloy surface and deposited thereon a composite coating, said composite coating comprising: a matrix formed by a first layer of an inorganic-based coating chemically bound directly to the at least one metallic magnesium or magnesium alloy surface, said matrix having pores and internal surfaces defined by the pores, at least some of said pores being in communication with an external surface of the first layer and forming openings therein; and a second component, different from the inorganic-based coating, applied to at least a portion of the matrix comprising the pores, said second component being in contact with at least some of the internal surfaces and external surfaces. In one embodiment the article further comprises a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating.

It is an object of the invention to provide an article having at least magnesium or magnesium alloy surface having chemically bonded thereto an electroceramic coating and, with or without intervening post-treatment, the electroceramic coating has adhered thereto a spray deposited, anti-fingerprint coating. The anti-fingerprint coating comprises uncoalesced spray droplets forming a series of flattened, cured globules which form a cured refractive surface.

In one embodiment of the invention an article is provided having an inorganic-based coating comprising a first sub-layer directly bonded to the bare, meaning a clean surface with no coating applied thereto, metallic magnesium or magnesium alloy surface at a first interface, the first sub-layer comprising Mg, O, C, P and at least one transition metal; and a second sub-layer integrally connected to the first sub-layer at a second interface, the second sub-layer comprising external surfaces at the outer boundary of the inorganic-based coating, and internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, the second sub-layer comprising Mg, O, C, P and the at least one transition metal and the second sub-layer having a composition such that the O exhibits a concentration gradient wherein O is greatest at the second interface and decreases to about 30-50 wt. % proximate to the external surfaces; and wherein each of Mg, P and C exhibit a concentration gradient in the second sub-layer such that concentration of each element in the second sub-layer is greatest proximate the external surfaces and decreases to its lowest concentration in the second-sub-layer proximate to the second interface.

For purposes of the invention, the term “inorganic-based coating” means that the electroceramic coating comprises substantial amounts of inorganic compounds and/or inorganic glasses, and the inorganic-based coating may additionally include some organic material, sourced from the raw materials, generated in situ or the like. Organic material will be understood to describe molecules that are made up of at least one carbon atom with hydrogen(s) bonded thereto, the carbons may form chains or cyclic structures and may optionally include additional atoms and functional groups (e.g. oxygen, silicon, phosphorus and nitrogen) attached. Generally, an inorganic-based coating may contain less than 50, 40, 30, 20, 10, 8, 6, 4, 2, 1 wt. %, the units more preferably being amounts in parts per thousand, most preferably parts per million, of organic material. The term “paint” includes all like materials that may be designated by more specialized terms such as lacquer, enamel, varnish, shellac, topcoat, and the like; and, unless otherwise explicitly stated or necessarily implied by the context. The simple term “metal” or “metallic” will be understood by those of skill in the art to mean a material, whether it be an article or a surface, that is made up of atoms of metal elements, e.g. magnesium, the metal elements present in amounts of at least, with increasing preference in the order given, 55, 65, 75, 85, or 95 atomic percent, the simple term “magnesium” includes pure magnesium and those of its alloys that contain at least, with increasing preference in the order given, 55, 65, 75, 85, or 95 atomic percent of magnesium atoms.

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, or defining ingredient parameters used herein are to be understood as modified in all instances by the term “about”. Throughout the description, unless expressly stated to the contrary: percent, “parts of’, and ratio values are by weight or mass; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise, such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; molecular weight (MW) is weight average molecular weight; the word “mole” means “gram mole”, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical or in fact a stable neutral substance with well-defined molecules; and the terms “storage-stable” is to be understood as including solutions and dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained at ambient room temperatures (18 to 25° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of elemental composition, in weight percent, of an inorganic-based electrolytically deposited coating according to the invention, measured by GDOS, showing varying chemical composition of a coating of the invention, as a function of distance from the magnesium alloy surface.

FIG. 2 is a drawing of a cross-section of a panel of AZ-31 coated according to Example 1, prior to post-treating, showing the inorganic-based coating and sub-layers thereof.

FIGS. 3a-3d show photographs of Comparative Examples 2, 3 and 4. FIG. 3a shows a photograph of an AZ-31 Mg alloy panel treated with a pH of 5.7 for 45 seconds and FIG. 3b shows a photograph of an AZ-31 Mg alloy panel treated with a pH of 6.6 for 45 seconds, both panels are bright shiny metallic in appearance with no coating deposited. FIG. 3c shows a photograph of an AZ-31 Mg alloy panel treated with a pH of 6.1 for 10 minutes, showing etching but no coating. FIG. 3d shows a photograph of an AZ-31 Mg alloy panel treated at pH 9.9 for 10 minutes, showing etching, but no coating.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Applicants have surprisingly discovered a way to make electroceramic coatings having a uniform darkly colored surface on magnesium, desirably without requiring subsequent smoothing of the coated surface. Articles according to the invention include magnesium-containing articles having a coating, which may be an electrolytically deposited coating, chemically bonded to one or more metal surfaces of the article, said coating having a dark appearance, e.g. black, brown, bronze, gray and the like, to the unaided human eye. Desirably the coatings show L*a*b* measurements in the following ranges: L=0 to 30 for black, and L*a*b* values corresponding to brown, bronze or gray as will be understood by those of skill in the art to be as defined by the International Lighting Commission (Commission Internationale de l'Eclairage, CIE) L*a*b* color space (1976) where L* indicates lightness, a* is the red/green coordinate, and b* is the yellow/blue coordinate.

Such articles are useful as for example, parts for motor vehicles, aircraft, and electronic devices, including handheld electronic devices, and other products where the light weight and strength of magnesium is desired. The articles generally have at least one metal surface, which comprises magnesium or magnesium alloy and chemically bonded directly to that metal surface is an inorganic-based coating. In some embodiments, the inorganic-based coating is post treated and/or painted.

At least a portion of the article has a metal surface that contains not less than 50% by weight, more preferably not less than 70% by weight, magnesium or magnesium alloy. The term “magnesium-containing article”, as used in the specification and the claims, means an article having at least one surface that may be in whole or in part metallic magnesium or a magnesium alloy. The body of the article may be formed of metallic magnesium or a magnesium alloy or may be formed of other materials, e.g. metals other than magnesium, polymeric materials, refractory materials, such as ceramics, that have a layer of magnesium or magnesium alloy on at least one surface. The other materials may be other metals different from magnesium, non-metallic materials or combinations thereof, such as composites or assemblies. The article may comprise at least one surface of metallic magnesium or a magnesium alloy comprising, in order of increasing preference, at least about 51, 60, 65, 70, 75, 80, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98 or 99 wt. % magnesium.

The coated metal surface has an appearance different from the uncoated metal surface. Desirably the coated metal surface may have a black, brown, bronze or gray colored appearance, and is generally darker than the bare metal surface and MOx coated surfaces where M is magnesium or magnesium alloying elements. The coating may have a uniform thickness or may be selectively deposited, e.g. using sealed chambers restricting electrolyte contact to only selected surfaces, masking and the like, such that the thickness of the coating is greater in selected areas of the metal surface.

Chemically bonded to the at least one metal surface of the article is a first layer comprising an inorganic-based coating. An inorganic-based coating may include some organic material, but contains a greater mass of inorganic material than of organic molecules. The inorganic material may act as a matrix in which any organic constituent may be distributed. In some embodiments, organic molecules may be absent. In some embodiments, carbon is present in the coating and organic molecules are not detected.

Desirably the inorganic-based coating may be applied by an electrolytic deposition process as described herein. In one embodiment, the inorganic-based coating contains carbon, oxygen, phosphorus, one or more transition metals, and magnesium. In one embodiment, the inorganic-based coating contains oxygen, at least one alloying element from the metal substrate in addition to at least one of magnesium or magnesium alloy, and at least one element from the bath. In another embodiment, the inorganic-based coating comprises carbon, oxygen, phosphorus, two or more transition metals, and magnesium. The foregoing may be assessed based on Glow Discharge Optical Emission Spectroscopy (GDOES), a spectroscopic method for the qualitative and quantitative analysis of metallic and non-metallic solid materials as is known in the art.

In some embodiments, despite the absence of organic or other carbonaceous components added to the electrolyte, the inorganic-based coating may comprise carbon. Both the carbon and alloying elements, if present, may be dispersed in a ceramic layer. Even with inclusion of carbon and alloying elements in the inorganic-based coating, a uniform thickness can be generated which provides uniform paint and adhesive bonding, as well as corrosion resistance, which is improved as compared to the bare surface of the magnesium containing substrate. This feature of the invention is beneficial in reducing scrap rate where substrates and the inorganic-based coatings deposited thereon achieve good coating quality even in the presence of carbon and alloying elements in the inorganic-based coating. It was surprisingly found that the corrosion performance remained substantially the same in the presence of carbon, which is often considered a contaminant indicating poor cleaning of the metal substrate. In one embodiment, the inorganic-based coating comprises C, O, P, Al, Mg, and at least one transition metal.

The inorganic-based coating may have a bi-layer morphology, as shown in FIG. 1 and FIG. 2. FIG. 1 is a graph of an elemental depth profile taken of inorganic-based coatings according to the invention using glow discharge optical emission spectroscopy (GDOES). Amounts of various elements are shown in weight percent at particular distances from the metal surface. FIG. 1 shows that the first sub-layer and the second sub-layer are different in morphology and elemental content.

FIG. 2 shows a cross-section of a magnesium alloy panel coated according to Example 1, prior to application of a post-treatment. The inorganic-based coating 100 has a bilayer structure, despite being deposited in a single processing step: a first sub-layer 120 directly bonded to the magnesium article 200 and having an interface 110 with the metal surface (first interface 110); and a second sub-layer 140 in direct contact with the first sub-layer and spaced away from the metal surface by the first sub-layer lying there between. The second sub-layer is directly bonded with the first sub-layer at an interface 130 with the first sub-layer (second interface). The second sub-layer of the inorganic-based coating comprises pores 160, and has internal surfaces 170 and external surfaces 150. The internal surfaces 170 are defined by pores 160 in the second sub-layer and lie interior to the outer boundary of the inorganic-based coating, which comprises the external surfaces 150 of the second sub-layer.

The external surfaces of the second sub-layer, lie in a boundary between inorganic-based coating and an external environment or a secondary layer applied to the outer boundary and are not in direct contact with a metallic surface of the magnesium-containing article. The first sub-layer may have few or no pores and has a more dense composition than the second sub-layer. Any pores present in the first sub-layer are desirably not contiguous between the metallic surface of the article and the external surface of the inorganic-based coating layer, and optionally smaller than the pores of the second sub-layer. Some of the pores of the second sub-layer are open pores in communication with the external surface. In some embodiments, the second sub-layer may comprise open and closed cell pore structure. Pore size may range from about 0.1 microns to 5 microns and may make up as much as 50% or more of the volume of the deposited coating. The electrolytically applied inorganic-based coating may have a surface area that is about 75-150× that of the uncoated substrate surface.

At least a portion of the inorganic-based coating has an amorphous structure. Physical morphology of the inorganic-based coating may comprise non-crystalline compounds of magnesium and one or more of elements. In one embodiment, the inorganic-based coating shows amorphous structure by X-ray crystallography (XRD). Desirably, the inorganic-based coating may be a hard (5-6 Moh hardness), amorphous coating comprising non-stoichiometric magnesium compounds. Nonstoichiometric glasses of Mg and transition metals as disclosed herein, with or without oxygen, may be present. In one embodiment the inorganic-based coating is an inorganic composition comprising Mg, O & Fe, including stoichiometric and non-stoichiometric compounds of said elements with each other. In another embodiment, the inorganic composition comprises crystalline and non-crystalline compounds comprising magnesium, with more than 50 atomic percent of the composition comprising non-crystalline compounds.

Coating thickness of the inorganic-based electrolytically deposited coating may range from 0.1 microns to about 50 microns, desirably 1-20 microns depending upon the desired use of the coated article. Coating thickness of the inorganic-based electrolytically deposited coating desirably is at least, in increasing order of preference 0.5, 1, 3, 5, 7, 9, 10 or 11 microns thick, and no more than, if only for economic reasons, in increasing order of preference, 50, 30, 25, 20, 15, 14, 13, or 12 microns thick. As a decorative layer, the coating may range from 2-5 microns. In one embodiment, the coating thickness ranges from 3 to 10 microns.

The Examples show that electrolytically applied inorganic-based coatings according to the invention perform better than commercially available conversion coatings for magnesium in unpainted and painted corrosion testing, as well as providing improved corrosion resistance when compared to PEO coatings on magnesium alloys typically used in the automotive industry, e.g. magnesium casting alloys and forged alloys. The electrolytically applied inorganic-based coatings perform better than commercially available conversion coatings for magnesium in unpainted and painted corrosion testing, as well as providing improved corrosion resistance when compared to PEO coatings on magnesium alloys typically used in the automotive industry, e.g. magnesium casting alloys and forged alloys.

In one embodiment, magnesium-containing article may have a composite coating wherein the inorganic-based coating may act as a matrix. This embodiment may include a coating comprising:

-   -   A) a matrix of a first layer of an inorganic-based coating         chemically bonded directly to a magnesium containing surface and     -   B) a second component that is different from the inorganic-based         coating and distributed throughout at least a portion of the         matrix.

In a further embodiment, the coating on the magnesium containing article may comprise:

-   -   A) a first layer of inorganic-based coating chemically bonded         directly to a magnesium containing surface,     -   B) a second component, e.g. a Ti, Zr or Hf or the like         post-treatment, that is different from the inorganic-based         coating and distributed throughout at least a portion of the         inorganic-based coating and     -   C) second layer that is different from the inorganic-based         coating and is adhered to at least external surfaces of the         inorganic-based coating,

In one embodiment of the invention, the second component may have the same composition as the second layer. In another embodiment of the invention, the second component may be different from both A) and C). In one embodiment, the second component and/or the second layer may form reaction products with elements in the inorganic-based coating. In one embodiment, the inorganic-based coating has a layer of paint deposited thereon, which may comprise the second layer or may be in addition to the second layer.

For a variety of reasons, it is preferred that inorganic-based coatings according to the invention, and aqueous compositions for depositing the inorganic-based coatings, as defined above, may be substantially free from many ingredients used in compositions for similar purposes in the prior art. Specifically, it is increasingly preferred in the order given, independently for each preferably minimized ingredient listed below, that aqueous compositions according to the invention, when directly contacted with metal in a process according to this invention, contain no more than 1.0, 0.5, 0.35, 0.10, 0.08, 0.04, 0.02, 0.01, 0.001, or 0.0002 percent, more preferably said numerical values in grams per liter, of each of the following constituents: chromium, cyanide, nitrite ions, organic surfactants, formaldehyde, formamide, urea, hydroxylamines, ammonia, tertiary amines, cyclic amines, e.g. hexamethylene tetraamine; silicon, e.g. siloxanes, organosiloxanes, silanes, silicate; rare earth metals; alkali metals, e.g. sodium, potassium; sulfur, e.g. sulfate; permanganate; perchlorate; boron, e.g. borax, borate; strontium, fluorine, e.g. free or bound fluoride; and/or free chloride. Also it is increasingly preferred in the order given, independently for each preferably minimized ingredient listed below, that as-deposited inorganic-based coatings and inorganic secondary layers according to the invention, contain no more than 1.0, 0.5, 0.35, 0.10, 0.08, 0.04, 0.02, 0.01, 0.001, or 0.0002 percent, more preferably said numerical values in parts per thousand (ppt), of each of the following constituents: chromium, cyanide, nitrite ions, organic surfactants, formaldehyde, formamide, urea, hydroxylamines, ammonia and hexamethylene tetraamine; silicon, e.g. siloxanes, organosiloxanes, silanes, silicate; rare earth metals; alkali metals, e.g. sodium, potassium; sulfur, e.g. sulfate; permanganate; perchlorate; boron, e.g. borax, borate; strontium, fluorine, e.g. free or bound fluoride; and/or free chloride.

Inorganic-based coatings can be produced by a variety of processes capable of generating hard, amorphous coatings chemically bonded to magnesium-containing metals. In one embodiment, the inorganic-based coating may be formed using electrolytic deposition according to the inventive process described herein.

For post-treatment, several commercially available options may be suitable including conversion coating compositions comprising fluorometallates of for example Ti, Zr Hf or combinations thereof. it has been found that suitable compositions for forming a second layer comprising organic polymer chains and/or inorganic polymer chains include, by way of non-limiting example aqueous compositions comprising (A) a component of a dissolved fluoroacids of one or more metals and metalloid elements selected from the group of elements consisting of titanium, zirconium, hafnium, boron, aluminum, germanium, and tin; and/or (B) a component of one or more of (i) dissolved or dispersed finely divided forms of metals and metalloid elements selected from the group of elements consisting of titanium, zirconium, hafnium, boron, yttrium, lithium, aluminum, germanium, and tin and (ii) the oxides, hydroxides, and carbonates of such metals and metalloid; plus (C) a component that is either (i) a water soluble or dispersible polymer and/or copolymer, preferably selected from the group consisting of (i,1) polymers and copolymers of one or more x-(N—R¹—N—²—aminomethyl)-4-hydroxy-styrenes, where x=2, 4, 5, or 6, R′ represents an alkyl group containing from 1 to 4 carbon atoms, preferably a methyl group, and R²—represents a substituent group conforming to the general formula H(CHOH)_(n) CH₂—, where n is an integer from 1 to 7, preferably from 3 to 5, (i.2) epoxy resins, particularly polymers of the diglycidylether of bisphenol-A, optionally capped on the ends with non-polymerizable groups and/or having some of the epoxy groups hydrolyzed to hydroxyl groups; (i.3) polymers and copolymers of acrylic and methacrylic acids and their salts; and (i.4) polymers and copolymers comprising silicon, which may be organic and/or inorganic polymers.

The treating may consist either of coating the surface of the first layer of an inorganic-based coating with a liquid film of the composition and then drying this liquid film in place on the surface of the first layer, or simply contacting the first layer of an inorganic-based coating with the composition for a sufficient time to produce an improvement in the resistance of the coated article to corrosion, and subsequently rinsing before drying. Such contact may be achieved by spraying, immersion, and the like as known per se in the art.

Depending on condition of the surface of the magnesium containing surface to be coated, the process may comprise optional steps of: cleaning, etching, deoxidizing and desmutting with or without intervening steps of rinsing with water. Where utilized, a rinse water may be counterflowed into a preceding bath. Prior to contacting the magnesium containing article with the electrolyte, a step 5) of masking or closing off portions of the article to limit or prevent contact with the electrolyte may be performed. For example, masking may be applied to magnesium containing portions of the article where no coating is desired or may be applied to protect article components or surfaces that might be damaged by the electrolyte, likewise hollow portions of an article, e.g. the lumen of a pipe, may be closed off or plugged to prevent electrolyte contact of interior surfaces.

Desirably between the steps of removing the coated article from the electrolyte and the posttreatment step the inorganic-based coating is not physically or chemically removed or etched. Specifically, no more than 1000, 500, 100, 50, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0.5 mg/m2 of the inorganic-based coating may be removed from the article. Preferably none of the deposited inorganic-based coating is removed.

As discussed above, there is no specific limitation on the article to be subjected to processing in accordance with the present invention, provided that the surface to be electrolytically coated has sufficient magnesium metal or other light metal in combination with magnesium, desirably in the zero oxidation state, to permit coating generation and the non-magnesiferous surfaces are not negatively affected by the treatments. Masking of selected surfaces to prevent contact with electrolyte can be accomplished by methods known in the art. The electrolytic treatment is advantageously applicable to magnesium-base alloys containing one or more other elements such as Al, Zn, Mn, Zr, Si and rare earth metals.

If electrolytic deposition is to be used, the magnesium containing surfaces to be coated are contacted with an alkaline electrolyte, as described herein. The electrolyte may have a pH of 10 or more, desirably greater than 10, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9 or 13. In carrying out the electrolytic deposition, an electrolyte is employed which may be maintained at a temperature between about 5° C. and about 90° C., desirably from about 20° to about 45° C.

The electrolyte is an alkaline solution or dispersion, which comprises; desirably consists essentially of; or optionally consists of water, an organic amine, a source of phosphorus, and at least one water soluble source of at least one transition metal, e.g. one or more additional components selected from: water-soluble transition metal oxides, water-soluble transition metal salts and mixtures thereof.

The organic amine is soluble or dispersible in the electrolyte. The organic amine may be a primary amine, desirably a monoamine, such as by way of non-limiting example monoethanolamine. Desirably the organic amine is present in the absence of cyclic amines or tertiary amines. Primary monoamines are preferred; secondary amines or diamines may be present provided that they do not interfere with deposition of the coating or corrosion resistance. The source of organic amine is generally present in an amount, in increasing order of preference, of about 50, 60, 70, 80, 90, 100, 105, 110, 115, 120, 125, 130, or 140 g/l and at most in increasing order of preference about 500, 400, 350, 300, 275, 250, 225, 200, 190, 180, 170, 160, 150, 145, 143 or 141 g/l.

Suitable sources of phosphorus include water-soluble acids and salts thereof, desirably oxy acids. The source may be inorganic or organic. Non-limiting examples include phosphoric acid, phosphorous acid, phosphonic acid, phosphates, pyrophosphate, phosphonates, and combinations thereof. The source of phosphorus is generally present in an amount, in increasing order of preference, of about 10, 15, 17, 19, 20, 21, 22, 23, 24, 25, 26, 28, 30, 32, 34, 36, 38 or 40 g/l and at most in increasing order of preference about 85, 80, 75, 70, 65, 60, 55, 50, 45, 44, 43, 42 or 41 g/l, calculated as PO_(x).

Water soluble sources of at least one transition metal include sources of the transition metal such as transition metal oxides, acids and salts of the metal oxides; non-oxidic transition metal salts and mixtures thereof. Salts may be inorganic or may include organic counterions. Examples of suitable sources include metal oxides e.g. oxides of vanadium and oxide salts thereof, acids and salts of the metal oxides include e.g. tungstic acid and ammonium metatungstate; and non-oxidic transition metal salts, e.g. iron citrate, iron acetate, iron acetylacetonate and the like; and combinations thereof. “Water-soluble” as used herein includes sources of the transition metals that may be insoluble or only slightly soluble in H₂O, but are soluble in the alkaline electrolyte as described herein. Preferred transition metals include iron, tungsten, vanadium and mixtures thereof. Suitable sources of iron are water soluble or alkali soluble salts of iron, such as by way of non-limiting example iron nitrate, iron sulfate, iron ammonium citrate, iron citrate, iron ammonium sulfate, iron acetate, iron acetylacetonate, and the like. Iron acetate and iron citrate are preferred. The source of transition metal is present as ions dissolved in the electrolyte, and the amount utilized in the electrolyte depends on the transition metal selected and the color desired. For black, each transition metal may be present in an amount up to the solubility limit of the transition metal ion, provided that amount present does not interfere with deposition of the coating, corrosion resistance or bath maintenance.

Desirably, iron ions may be present in an amount, in increasing order of preference, of about 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, 1.1, 1.2, 1.3, 1.4 g/l and at most in increasing order of preference about 5.0, 4.0, 3.5, 3.0, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, 1.8, 1.7, 1.6, or 1.5 g/l.

Desirably, vanadium may be present in an amount, in increasing order of preference, of about 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, 1.1, 1.2, 1.3, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, or 1.85 g/l and at most in increasing order of preference about 10, 9,8,7,6, 5.5, 5.0, 4.5, 4.25, 4.0, 3.9, 3.8, 3.7, 3.6, 3.5, 3.4, 3.3, 3.2, 3.1, 3.0, 2.9, 2.8, 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.15, 2.1, 2.05, 2.0, 1.95, 1.9 or 1.875 g/l.

Desirably, tungsten may be present in an amount calculated as tungstic acid, in increasing order of preference, of about 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9.00, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.00, 10.1, 10.2, 10.3, 10.45g/l and at most in increasing order of preference about 20, 19, 18, 17, 16, 15, 14, 13, 12, 12.5, 12.4, 12.3, 12.2, 12.1, 12.0, 11.9, 11.8, 11.7, 11.6, 11.5, 11.4, 11.3, 11.2, 11.1, 11.0, 10.9, 10.8, 10.7, 10.6, or 10.5 g/l.

Optionally, the electrolyte may contain at least one additive such as a ligand, a chelant or the like capable of forming coordination complexes with the transition metals in the electrolyte bath for example acetylacetone.

In one embodiment, the organic amine is monoethanolamine and the at least one transition metal element comprises one or more of iron, vanadium and tungsten. In one embodiment, the alkaline electrolyte contains less than 100 ppm silicon or aluminum and is essentially free of fluorine and tertiary amines. In one embodiment, the organic amine is a primary monoamine in the absence of cyclic amines, and the at least one transition metal element consists of iron or vanadium or tungsten. In one embodiment, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and vanadium and the alkaline electrolyte has a pH of at least 10.2. In one embodiment, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises tungsten. In one embodiment, the alkaline electrolyte is vanadium free, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and optionally a second transition metal element other than vanadium. In one embodiment, the organic amine is a primary monoamine in the absence of cyclic amines, and the at least one water-soluble or dispersible source of at least one transition metal element comprises iron citrate.

The composition may be provided as a storage-stable two pack system wherein Part A contains water; a source of phosphorus, for example phosphoric acid, phosphorous acid, pyrophosphate, phosphonate; one or more water soluble salts of transition metals, for example iron, vanadium, tungsten and the like; wherein the mass ratio of phosphorus to total amount of transition metal is 4:1 to 1:1; and Part B contains organic amine, preferably monoethanolamine, Part A and Part B being provided in amounts such that the mass ratio of Part A to Part B be ranges from 1:1 to 2:1.

In one embodiment, a method is provided wherein a magnesium or magnesium alloy surface is contacted with, desirably immersed in, an aqueous electrolyte and electrolyzed as the anode in the circuit. One such process comprises immersing at least a portion of the article in the electrolyte, which is preferably contained within a bath, tank or other such container. A second article that is cathodic relative to the anode is also placed in the electrolyte. Alternatively, the electrolyte is placed in a container which is itself cathodic relative to the article (anode). Voltage is applied across the anode and cathode for a time sufficient to form an inorganic-based electrolytic coating. The time required to produce a coating in an electrolytic process according to the invention may range from about 30, 60, 90, 120 seconds, up to about 150, 180, 210, 240, 300 seconds. Longer deposition times may be utilized but are considered commercially undesirable. Electrolytic processing time can be varied to maximize efficiency by reducing time to Vmax and to control coating weight.

Alternating current, direct current or a combination may be used to apply the desired voltage, e.g. straight DC, pulsed DC, AC waveforms or combinations thereof. In one embodiment, pulsed DC current is used. Desirably a period of at least 0.1, 0.5, 1.0, 3.0, 5.0, 7.0, 9.0, or 10 millisecond and not more than 50, 45, 40, 35, 30, 25, 20, or 15 millisecond may be used, which period may be held constant or may be varied during the immersion period. Waveforms may be rectangular, including square; sinusoidal; triangular, sawtooth; and combinations thereof, such as by way of non-limiting example a modified rectangle having at least one vertical leg that is not perpendicular to the horizontal portion of the rectangular wave.

Peak voltage potential desirably may be, in increasing order of preference, up to about 800, 700, 600, 500, 400 volts, and may desirably is at least in increasing order of preference 200, 250, 300, 350, 375 or 395 volts. Lower voltages generate thinner films that are generally lighter in color, which may be acceptable for gray or to obtain a tan color.

Average voltage may be in increasing order of preference at least 300, 310, 320 330, 350, or 375 volts and independently preferably may be less than 600, 550, 500, 450, 425 or 400 volts. In one embodiment, average voltage can range from about 300-450 volts. In another embodiment, average voltage may be selected to be in a higher range of 400-550 volts.

Voltage is applied across the electrodes until a coating of the desired thickness is formed on the surface of the article. Generally, higher voltages result in increased overall coating thickness. Higher voltages may be used within the scope of the invention provided that the substrate is not damaged and coating formation is not negatively affected.

Prior to electrolytic coating, magnesium-containing surfaces may be subjected to one or more of cleaning, etching, deoxidizing and desmutting steps, with or without rinsing steps. Cleaning may be alkaline cleaning and a cleaner may be used to etch the surfaces. A suitable cleaner for this purpose is Parco Cleaner 305, an alkaline cleaner commercially available from Henkel Corporation. Desirably, the magnesium-containing surfaces may be etched by at least in increasing order of preference 1, 3, 5, 7, 10, or 15 g/m2 and independently preferably, at least for economy, not more than 20, 25, 30, 35, 40, 45 or 50 g/m2. Etching can be accomplished using commercially available etchants and/or deoxidizers for magnesium. Depending on the magnesium or magnesium alloy composition and cleanliness, a desmutting step may also be included in processing. Suitable desmutters include acids such as carboxylic acids, e.g. hydroxyacetic acid, alone or in combination with chelators and nitrates. If any of the above-described steps is utilized, the magnesium-containing surfaces are typically rinsed as a final step to reduce introduction of the prior steps' chemistries into the electrolyte.

Additional processing steps may be used after deposition of the inorganic-based coating, such as rinsing with water, alkaline solutions, acid solutions and combinations of such steps. In some embodiments, the process may include steps of applying at least one post-treatment, which may be dispersed in the inorganic-based coating, may form reaction products therewith, and/or may form an additional layer and combinations thereof. The additional layer may be an inorganic layer, an organic layer or a layer that comprises inorganic and organic components. Advantageously, any post-treatments, including for example additional layers described herein, are durably bound to the inorganic-based coating; while other removable layers for masking during manufacture or for shipping after coating may be applied.

The porous structure of the electrolytically deposited inorganic-based coatings on the magnesium containing article was a particular challenge for post-treatments that are not pore closing due to the significant surface area present on the internal surfaces of inorganic-based coatings. Surface area of inorganic-based coatings according to the invention is generally 75 to 100 times that of the original metal surface, by BET measurement. Such surface area is typically not found in conventional conversion coatings. The Ti, Zr and the like -containing post-treatment step, described above was surprisingly found to be a suitable method for introducing a second component for additional corrosion protection, in processes according to the invention, despite other post-treatments useful for anodized layers having little or no positive effect on corrosion resistance. For example, conventional post-treatments for anodized magnesium, including nickel based salts and lithium salts were found to provide insufficient unpainted corrosion resistance. In contrast, post-treatment of the inorganic-based coating with a fluorometallate-containing composition provided improvements in corrosion resistance. The fluorometallate containing post-treatment step may be used immediately after deposition of the inorganic-based coating, which may be dried.

At least one post-treatment composition is desirably introduced to the second sub-layer of inorganic-based coating, contacting at least the external surfaces and desirably at least some of the internal surfaces thereof. The second component may comprise the post-treatment composition and/or may comprise reaction products of the post-treatment composition and elements of the inorganic-based coating. In one embodiment, the post-treatment composition reacts with elements of the inorganic-based coating to thereby form a second component, which is different from the inorganic-based coating at least in that the second component comprises a metal or polymer from the posttreatment. The second component may form a thin inorganic-based coating in contact with the external surfaces of the inorganic-based coating and lining at least a portion of the pores in the inorganic-based coating.

In some embodiments, the post-treatment compositions may also contact internal surfaces of the inorganic-based coating and/or react with elements of the internal surfaces rendering the inorganic-based coating more resistant to corrosion producing agents reaching the magnesium containing surface. Depth of penetration of second components into the inorganic-based coating matrix may include up to 70, 65, 60, 55 or 50% of total thickness of the porous second sub-layer of the inorganic-based coating, said total thickness being measured from the second interface to the external surface of the inorganic-based coating.

In some embodiments, the post-treatment composition may be reactive with elements in the inorganic-based coating. Contacting the inorganic-based coating with a post-treatment composition provides improved corrosion resistance and does not cover up the pores in external surfaces of the inorganic-based coating. This is beneficial if a subsequent paint step is to be used because the pores provide anchoring sites for adhering paint to the surface.

Another post-treatment step which may be employed is depositing an additional layer comprising a polymer, preferably this may be done using a thermosetting resin which may or may not react with the inorganic-based coating. Average thickness of the polymeric second layer, as measured from an external surface of the inorganic-based coating to an outer surface of the second layer, may range from, in order of increasing preference, at least about 0.1, 0.25, 0.5, 0.75, 1, 2, 3, 4, or 5 microns and in order of increasing preference, not more than about 14, 12, 10, 8 or 6 microns. In contrast, typical paint thicknesses are at least 25 microns thick. Use of either a thin polymeric layer, as described above, or a paint, generally covers the pores in the external surfaces of the inorganic-based coating, the pores providing improved adhesion of the polymer or paint and surprisingly resulting in a uniform surface.

Desirably, polymers forming the second layer may comprise organic polymer chains or inorganic polymer chains. Examples of polymers suitable for an additional layer include by way of non-limiting example, silicone, epoxy, phenolic, acrylic, polyurethane, polyester, and polyimide. In one embodiment, organic polymers selected from epoxy, phenolic and polyimide are utilized. Preferred polymers forming additional layers include phenol-formaldehyde-based polymers and copolymers generated from, for example novolac resins, which have a formaldehyde to phenol molar ratio of less than one, and resole resins having a formaldehyde to phenol molar ratio of greater than one. Such polyphenol polymers can be made as is known in the art for example according to U.S. Pat. No. 5,891,952. Novolac resins are desirably used in combination with a crosslinking agent to facilitate curing. In one embodiment, a resole resin having a formaldehyde to phenol molar ratio of about 1.5 is utilized to form a polymer additional layer on the inorganic-based coating. Phenolic resins useful in forming polymeric layers desirably have molecular weights of about 1000 to about 5000 g/mole, preferably 2000 to 4000 g/mole.

At least one of the above-described resins is desirably introduced to the first layer of inorganic-based coating, contacting at least the external surfaces thereof, and crosslinking to thereby form a polymeric layer on external surfaces of the inorganic-based coating. This polymeric second layer is different from the inorganic-based coating and is adhered to the inorganic-based coating.

In some embodiments, the resin may also contact internal surfaces of the inorganic-based coating and upon curing form a polymeric second component that is different from the inorganic-based coating and distributed throughout at least a portion of the inorganic-based coating. Analysis of inorganic-based coatings according to the invention that have been contacted with a resole resin having a formaldehyde to phenol molar ratio of 1.5 showed the polymeric components present in the inorganic-based coating matrix thereby forming a composite coating. Depth of penetration of polymeric second components into the inorganic-based coating matrix may range in increasing order of preference from 1, 2, 5, 10, 15, 20 or 25% and in increasing order of preference may be not more than 70, 65, 60, 55 or 50, 45, 40 or 35% of total thickness of the inorganic-based coating, said total thickness being measured from the first interface to the external surface of the inorganic-based coating.

In some embodiments, the resin may comprise functional groups reactive with elements in the inorganic-based coating, which may form bonds between the resin and the inorganic-based coating. For example, uncured novolac and resole resins comprise OH functional groups which may react with metals in the inorganic-based coating thereby linking the polymer to the inorganic-based coating.

Coated substrates according to the invention are useful in motor vehicles; aircraft and electronics where the combination of the inorganic-based coating and post-treatment layers can provide more corrosion protection than paint or anodizing alone, while ceramic-type hardness of the combination imparts additional toughness to external layers because sharp objects have greater difficulty in deforming a harder substrate prime layer than magnesium, which is relatively soft as compared to ceramic. Coatings according to the invention also can be beneficial in keeping the topcoat gloss and color readings relatively consistent by providing a relatively uniform paint base.

The process and coated articles of the invention provide a more uniform dark colored surface on magnesium and magnesium alloys, by way of non-limiting example AZ-31B, AZ-91B, AZ-91D, AM-60, AM-50, AM-20, AS-41, AS-21, AE-42, LZ-91, WS-82 and AM-lite®, a proprietary Mg—Zn—Al alloy. The uniformity aids in adhesion of any subsequently applied layers which provides improved corrosion resistance.

EXAMPLES

Commercially available magnesium or magnesium alloy test panels were utilized for all examples. The AZ-31 Mg alloy panels were about 93-97 wt.% Mg, the remainder being made up of Al, Zn, Mn, and less than 0.5 wt. % of other metal and metalloid impurities. The AZ-91 Mg alloy panels had less magnesium, about 87-91 wt. % Mg, with the remainder being made up of Al, Zn, Mn, and less than 1.2 wt. % of other metal and metalloid impurities.

Cleaning Steps:

All AZ-31 panels were cleaned in 5% BONDERITE® C-AK 305, an alkaline cleaner commercially available from Henkel Corp., at 60° C. for 3 minutes; rinsed with deionized water (Deionized water); deoxidized in 3% BONDERITE® C-IC HX-357 at 20-22° C. for 90 seconds, which was about a 30 g/m² etch rate.

All AZ-91 panels were cleaned with Turco® 6849 alkaline cleaner for one minute; rinsed with Deionized water; deoxidized with a commercially available phosphate based deoxidizer at 20-22° C. for 60 seconds; desmutted with a 25,000 KHz ultrasound bath of 1 gram per liter citric acid.

Coating Conditions

The conditions for electrolytic coating process for the Examples, unless stated otherwise: bath temperature was maintained between 20-25° C., the panels were immersed in the electrolyte as the anode and the cathode was steel. After coating and removal from the electrolyte, the coated panels were rinsed with deionized water. The coated panels were allowed to dry and were not baked, kilned, calcined or otherwise heat treated at temperatures greater than 100° C.

Example 1 BLACK Electroceramic Coating On Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath containing:

TABLE 1 Electrolyte Components (g) Deionized water 858.0 Phosphoric acid 75% 26.0 Iron (II) acetate 7.0 g Acetylacetone 5.0 g Monoethanolamine 100.0 Ammonium metavanadate 4.0

The electrolyte pH was measured at 10.55. The panels were electrolytically coated as the anode for 30 seconds at a peak voltage of 350 volts utilizing a square DC waveform of 25 milliseconds on and 9 milliseconds off generating an edge-covering, inorganic-based coating. The coated panels were removed from the electrolyte bath and rinsed with Deionized water for 300 sec.

The resulting coating appeared uniformly black to the unaided human eye. Color was measured with a Minolta Cr 300 color meter: the coating had color values of L, a, b of: 29.93, −2.14 and +2.33 respectively. The inorganic-based coating had a uniform texture and surface appearance. The coating thickness was measured and had a thickness of 10.01 microns. Coating was corrosion tested under e-coat paint and passed 504 hours of B-117 ASTM NSS testing.

Example 2 BLACK Electroceramic Coating On Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath, as described below, and coated under the same conditions as Example 1:

TABLE 2 Electrolyte Components (g) Deionized water 855.0 Phosphoric acid 75% 26.0 Iron (III) citrate 10.0 Acetylacetone 5.0 g Monoethanolamine 100.0 Ammonium metavanadate 4.0 The resulting coating appeared uniformly black to the unaided human eye. The coating was measured with a Minolta Cr color meter and had an L, a, b of: 28.18, −2.68, +2.33 respectively, the inorganic-based coating had a uniform texture and surface appearance, and the coating thickness was measured to be 10.22 microns.

Example 3 BLACK Electroceramic Coating On Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath, as described below, and coated under the same conditions as Example 1:

TABLE 3 Electrolyte Components (g) Deionized water 864.0 Phosphoric acid 75% 26.0 Iron (II) acetate 5.0 Monoethanolamine 100.0 Ammonium metavanadate 5.0 The pH of the bath was measured at 10.26. The resulting coating appeared uniformly black to the unaided human eye. The resulting coating was measured with a Minolta Cr color meter and had an L, a, b of: 28.87, −2.66 and +2.70 respectively. The inorganic-based coating had a uniform texture and surface appearance and had a thickness of 9.29 microns.

Example 4 BLACK Electroceramic Coating On Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath, as described below, and coated under the same conditions as Example 1:

TABLE 4 Electrolyte Components (g) Deionized water 864.0 Phosphoric acid 75% 26.0 Iron (III) acetylacetonate 5.0 Monoethanolamine 100.0 Ammonium metavanadate 5.0

The pH of the bath was measured at 10.30. The resulting coating appeared uniformly black to the unaided human eye. The coating was measured with a Minolta Cr color meter and had an L, a, b of: 32.59, −1.62 and +6.13 respectively. The inorganic-based coating had a uniform texture and surface appearance and had a thickness of 10.06 microns.

Example 5 BROWN Electroceramic Coating on Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath, as described below, and coated under the same conditions as Example 1:

TABLE 5 Electrolyte Components (g) Deionized water 790.0 Phosphoric acid 75% 50.0 Iron (III) citrate 5.0 Monoethanolamine 150.0 Ammonium metavanadate 5.0

The pH of the bath was measured at 10.33. The resulting coating appeared uniformly brown to the unaided human eye. The coating was measured with a Minolta Cr color meter and had an L, a, b of: 42.19, −1.89 and +11.90 respectively. The inorganic-based coating had a uniform texture and surface appearance and had a thickness of 15.76 microns.

Example 6 BLACK Electroceramic Coating On Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath according to Example 3, and coated under the same conditions as Example 1, except a higher peak voltage of 420 volts was used. The resulting coating was measured with a Minolta Cr color meter and had an L, a, b of: 25.21, −2.75 and +1.55 respectively. The inorganic-based coating was uniform and had a thickness of 14.98 microns.

Example 7 Comparative Example 1

AZ-31 Mg alloy panels were cleaned with the same cleaning used in Example 1. Thereafter, the panels were immersed in an electrolyte bath and subjected to plasma electrolytic oxidation (PEO) coating in the high concentration phosphate electrolyte, as described below, with ammonium metavanadate (NH₄ VO₃):

TABLE 6 Electrolyte Components (g/L) KOH 28.06 Ammonium metavanadate 9.36 Potassium pyrophosphate 165.17

The panels were processed under conditions, including pH, conductivity, composition and electrical input, identical to Bath B in Table 1 as disclosed in “Effect Of Ammonium Metavanadate On Surface Characteristics Of An Oxide Layer Formed On Mg Alloy Via Plasma Electrolytic Oxidation” Surface & Coatings Technology, Volume 236, 15 Dec. 2013, Pages 70-74. Despite processing according to the example of the above-identified publication, no coating was observed over 90% of the test panel. On portions of the test panels that were not completely bare and shiny, no measurable coating was found.

Example 8 GRAY Electroceramic Coating On Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath, as described below, and coated under the same conditions as Example 1:

TABLE 7 Electrolyte Components (g) Deionized water 790.0 Phosphoric acid 75% 26.0 Tungstic acid 10.0 Monoethanolamine 150.0

The resulting coating was a uniform grey color. The inorganic-based coating had a thickness of 10.2 microns. The resulting coating was measured with a Minolta Cr color meter and had an L=53.83, a=−3.45 and b=+6.72.

Example 10 BLACK Electroceramic Coating On Magnesium

AZ-31 Mg alloy panels were immersed in an electrolyte bath, as described below, and coated under the same conditions as Example 1:

TABLE 8 Electrolyte Components (g) Deionized water 814.0 Phosphoric acid 75% 26.0 Iron (III) citrate 5.0 Monoethanolamine 150.0 Ammonium metavanadate 5.0

The electrolyte pH was measured at 10.51. The resulting coating was measured with a Minolta Cr color meter and had an L=28.69, a=−2.60, b=+2.30. The inorganic-based coating was extremely uniform and had a thickness of 8.48 microns.

This coating had emissivity of 0.77 (on a scale of 0 to 1) which makes it useful for heat sink and heat dissipation applications, for example in electronics where light weight and uniform visual appearance are desirable.

GDOES depth profile was performed for this example. The GDOES results, shown in FIG. 1, tend to show that the chemical composition in the outermost 0.5 microns of the electroceramic coating makes a significant contribution to imparting a desired color to the coating.

Example 11 Comparative Examples 2, 3 and 4

Three comparative examples (Comparative Examples 2, 3 and 4) were run on magnesium panels using electrolytes analogous to “Exemplified Embodiment 1”, “Exemplified Embodiment 4” and “Exemplified Embodiment 6” of US Patent Publication No. 2015/0083598. The electrolytes were made using deionized water as the solvent, with resulting pH's being 5.7, 9.9 and 6.6 respectively. The pH of these electrolytes was selected to span the range of pH values disclosed in the '598 publication, to test how the '598 electrolytes performed on magnesium, which is claimed, but not exemplified in this publication.

Process

A standard commercial cleaning and deoxidizing process was used to prepare AZ31 panels which was identical in panels and preparation to the examples according to the invention. Panels measuring 1 inch by 2 inches (2.54 cm by 5.08 cm) were each immersed in one of the electrolytes and electrolyzed at 20 amps, 350 volts utilizing a square DC waveform of 25 milliseconds on and 9 milliseconds off, for 45 secs. Amperage and voltage were within the parameters disclosed in the '598 publication and time was selected to be comparable to the examples according to the invention.

The AZ31 panels were removed from the electrolyte, allowed to dry and examined. The panels showed etching of the magnesium, but no coating layer.

Additional AZ31 panels were run in the electrolytes with longer contact times and once again only etched magnesium panels were produced. Photographs of the panels after processing as described above are shown in FIGS. 3a and 3b , the shiny, silvery appearance clearly shows that neither sample had a black coating or any visible coating deposited.

The process parameters were adjusted, namely an electrolyte analogous to Exemplary Embodiment 1 with a pH of 6.1 to match that of Exemplary Embodiment 1 was made by adding an additional 17 grams of hexamethylenetetramine. Re-ran baths analogous to “Exemplified Embodiment 1”, “Exemplified Embodiment 4” for 10 minutes each using the same voltage and waveform as the examples according to the invention and otherwise process parameters described above from the '598 publication. Neither bath produced a dark or black coating. Photographs of the panels after processing at pH 6.1 and pH 9.9 for 10 minutes, as described above, are shown in FIGS. 3c and 3d , respectively; neither sample had a black coating deposited on the panel. A hazy, non-uniform, white appearing discoloration can be seen on portions of the panel. These tests tend to show that the process disclosed in the '598 publication does not provide a uniform black, brown, bronze or grey coating on magnesium.

The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims. 

What is claimed is:
 1. A method of depositing a dark colored coating on magnesium or magnesium alloy metal surfaces comprising: A) providing an alkaline electrolyte and a cathode in contact with the alkaline electrolyte; the alkaline electrolyte, which may be a solution or dispersion, comprising water, an organic amine, a source of phosphorus, and at least one water-soluble or water-dispersible source of at least one transition metal element; B) placing an article having at least one metallic magnesium or magnesium alloy surface in contact with the electrolyte and electrically connected thereto such that the surface acts as an anode; C) passing a current between the anode and cathode through the electrolyte solution for a time effective to generate a first layer of an inorganic-based coating chemically bonded directly to the magnesium or magnesium alloy metal surface, the first layer appearing black, brown, bronze or gray to the unaided human eye; D) removing the article having the at least one metallic magnesium or magnesium alloy surface coated with the first layer of an inorganic-based coating from the alkaline electrolyte and optionally rinsing and drying the article; E) optionally post-treating at least the first layer of the inorganic-based coating by 1) contacting the first layer of the inorganic-based coating with a post-treatment composition different from the inorganic-based coating, the post-treatment composition optionally being reactive with the inorganic-based coating; and/or 2) after step 1) if present, applying to the first layer of the inorganic-based coating, a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains having a layer thickness of 0.1 micron to 15 microns; and F) optionally applying a layer of paint after the post-treating step.
 2. The method of claim 1, wherein said method is performed in the absence of any step prior to step B) that deposits a material containing silicon and/or fluorine on the magnesium surface.
 3. The method of claim 1, wherein the organic amine is monoethanolamine and the at least one transition metal element comprises one or more of iron, vanadium and tungsten.
 4. The method of claim 1, wherein the alkaline electrolyte contains less than 100 ppm silicon or aluminum and is essentially free of fluorine and of tertiary amines.
 5. The method of claim 1, wherein the organic amine is a primary monoamine in the absence of cyclic amines, and the at least one transition metal element consists of iron or vanadium or tungsten.
 6. The method of claim 3, wherein the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and vanadium and the alkaline electrolyte has a pH of at least 10.2.
 7. The method of claim 3, wherein the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises tungsten.
 8. The method of claim 1, wherein the alkaline electrolyte is vanadium free, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and optionally a second transition metal element other than vanadium.
 9. The method of claim 1, wherein the organic amine is a primary monoamine in the absence of cyclic amines, and the at least one water-soluble or dispersible source of at least one transition metal element comprises iron citrate.
 10. The method of claim 1, further comprising performing at least one step selected from cleaning, etching, deoxidizing, desmutting, and combinations thereof prior to placing the magnesium containing article in contact with the alkaline electrolyte such that prior to generating the first layer, from 0.05 to 50 g/m² of metal is removed from the bare metallic magnesium or magnesium alloy surface.
 11. The method of claim 1, further comprising a step of masking a portion of the magnesium containing article prior to placing the at least one metallic magnesium or magnesium alloy metal surface in contact with the alkaline electrolyte.
 12. The method of claim 1, comprising controlling temperature and concentration of the alkaline electrolyte and providing a selected waveform of the current in step E) for a time sufficient to thereby produce the inorganic-based coating at a thickness of 1-40 microns and forming the first layer in step E) utilizes less than 10 kWh per square meter of the metal surface coated.
 13. The method of claim 1, wherein after step E), no more than 10 mg/m² of the inorganic-based coating is removed.
 14. The method of claim 12, wherein said current is pulsed direct current having an average voltage in a range of 50 to 700 volts.
 15. An article comprising at least one magnesium or magnesium alloy metal surface coated according to claim
 1. 16. An article comprising at least one metallic magnesium or magnesium alloy surface coated with a dark-colored first layer of an inorganic-based coating chemically bonded directly to the at least one metallic magnesium or magnesium alloy surface wherein the inorganic-based coating has a bilayer structure, comprising: a. a first sub-layer directly bonded to the metallic magnesium or magnesium alloy surface at a first interface, said first sub-layer comprising Mg, O, C, P and at least one transition metal element; b. a second sub-layer integrally connected to the first sub-layer at a second interface, said second sub-layer comprising external surfaces at an outer boundary of the inorganic-based coating, and optionally internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, said second sub-layer comprising the Mg, O, C, P and at least one transition metal element; wherein the weight percent of C in the second sub-layer is greater than that of the first sub-layer.
 17. The article of claim 16, where the weight percent of C in the second sub-layer exhibits an increasing concentration gradient from the second interface to the external surfaces of the inorganic-based coating.
 18. An article having at least one metallic magnesium or magnesium alloy surface and deposited thereon a composite coating, said composite coating comprising: a. a matrix formed by a first layer of an inorganic-based coating chemically bound directly to the at least one metallic magnesium or magnesium alloy surface, said matrix having pores and internal surfaces defined by the pores, at least some of said pores being in communication with an external surface of the first layer and forming openings therein; and b. a second component, different from the inorganic-based coating, applied to at least a portion of the matrix comprising the pores, said second component being in contact with at least some of the internal surfaces and external surfaces.
 19. The article of claim 18, further comprising a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating.
 20. The article of claim 16, further comprising a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating.
 21. A plasma electrolytic deposition electrolyte comprising: an aqueous alkaline solution or dispersion, which comprises water, a water-soluble or dispersible organic amine present in an amount of about 50 to about 500 g/l, a source of phosphorus selected from water-soluble oxy acids and salts thereof present in an amount of about 10 to about 85 g/l, and at least one water-soluble or dispersible source of at least one transition metal.
 22. The plasma electrolytic deposition electrolyte of claim 21, wherein the aqueous alkaline solution or dispersion contains less than 100 ppm silicon or aluminum and is essentially free of fluorine and tertiary amines.
 23. The plasma electrolytic deposition electrolyte of claim 21, wherein the organic amine is a primary monoamine in the absence of cyclic amines, and the at least one transition metal element consists of iron or vanadium or tungsten.
 24. The plasma electrolytic deposition electrolyte of claim 23, the at least one water-soluble or dispersible source of at least one transition metal element comprises iron citrate.
 25. The plasma electrolytic deposition electrolyte of claim 21, wherein the organic amine is monoethanolamine and the at least one transition metal element comprises one or more of iron, vanadium and tungsten.
 26. The plasma electrolytic deposition electrolyte of claim 21, wherein the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and vanadium and the alkaline electrolyte has a pH of at least 10.2.
 27. The plasma electrolytic deposition electrolyte of claim 21, wherein the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises tungsten.
 28. The plasma electrolytic deposition electrolyte of claim 21, wherein the electrolyte is vanadium free, the organic amine is monoethanolamine, the source of phosphorus is phosphoric acid, and the at least one transition metal element comprises iron and optionally a second transition metal element other than vanadium.
 29. A storage-stable two pack system comprising: a. a Part A containing water; a source of phosphorus; one or more water soluble salts of transition metals, said transition metals comprising iron, vanadium and/or tungsten; wherein Part A has a mass ratio of phosphorus to total amount of transition metal from 4:1 to 1:1; and b. a Part B containing organic amine; wherein Part A and Part B are provided in amounts such that the mass ratio of Part A to Part B is in a range from 1:1 to 2:1. 